Current observations of supermassive black holes in the early Universe pose serious questions on our understanding of the formation and growth of these unique astrophysical objects. Dr. Dimitrios Psaltis (University of Arizona) will determine whether supermassive black holes can grow in the early Universe solely by accretion. This will be achieved by simulating accretion flows onto black holes using the spectral, 3D, magnetohydrodynamic algorithm recently developed by Dr. Psaltis and his group, which will be further enhanced by incorporating a detailed treatment of radiative processes. The result of this investigation will be the first ab initio calculation of the limiting rate of mass accretion by a supermassive black hole. This project will also assess the viability of performing tests of gravity using accreting black holes. Moreover, it will establish, using a parametric theoretical framework, whether the astrophysical black holes are described by the Kerr metric of General Relativity.
An integral part of this research project will be the development of modules for the visualization of particle and photon orbits in arbitrary curved spacetimes. These modules will become valuable tools in educating students in the various geometric aspects of the theory. Dr. Psaltis, assisted by several undergraduate students, will develop a self contained, web-based, visual tool for the teaching of relativistic concepts. Two graduate students and several undergraduate students will be educated during the research program in various aspects of relativity, computational astrophysics, and transport methods, both of which are important for a number of disciplines of physics research other than astrophysics.
The project supported by this award had three key goals: (i) to assess the viability of performing tests of gravity using accreting black holes. (ii) to establish, using a parametric theoretical framework, whether the astrophysical black holes are described by the Kerr metric of General Relativity, and (iii) to develop a self contained, web-based, visual tool for the teaching of relativistic concepts. A remarkable progress has been achieved in all three key aspects outlined in the proposal. In particular: (i) We demonstrated that, even within the complexities of astrophysical observations, accreting black holes are very useful object in performing accurate, quantitative tests of strong-field general relativity. In particular, we explored the astrophysical implications of particular braneworld gravity models on the dynamics and evolution of black-hole binaries and demonstrated that observable effects can constrain the microscopic parameters of such extensions to GR. (ii) We developed a parametric framework that quantifies potential deviations of observable black-hole spacetimes from the Kerr solution of GR based on the no-hair theorem.We developed a new, fast ray tracing algorithm for general spacetimes, which we later applied on various projects in neutron-star astrophysics. (iii) We developed a web-based set of apps that allow for the visualization of a number of important concepts in general relativity, including but not limited to the properties of horizons in different coordinate systems, the properties of orbits in GR spacetimes, and graviational lensing. These web sites have been used consistently in graduate and undergraduate classes at the UofA. Within this project, two graduate students were educated in different aspects of general relativity, black-hole physics, cosmology, and high-performance computing. These two student have state-of-the art knowledge in all these fields, which they can now use to develop their own research and teaching programs. The web-based teaching tools for GR visualization have been valuable teaching tools in the education of more than one hundred undergraduate and graduate students at the Physics, Astronomy, and Math departments at the University of Arizona. The tools helped the students understand important relativistic concepts and develop their intuition in problem solving. The new framework for testing General Relativity with astrophysical black holes that we developed, i.e., the test of the GR no-hair theorem, has become one of the key driving forces behind the NSF funded Event Horizon Telescope project. The main goal of the Event Horizon Telescope is to generate horizon-scale images in the mm of at least two black holes, the black hole in the center of the Milky Way and the one in the center of the galaxy M87. The first-order qualitative result of the Event Horizon Telescope is the presence of the black-hole shadow, imprinted on the emission from the accretion flow that surrounds the black hole. However, the key quantitative relativsitic result of the Event Horizon Telescope will be the measurement of potential deviations (or just upper limits) from the predictions of the relativistic no-hair theorem, as explored in the articles our group was published within this award.
